1. Field of the Invention
The invention relates to a radiant element, particularly for glass ceramic hotplates, with an electrical light radiator and at least one series resistor.
2. Prior Art
Such radiant elements are e.g. known from U.S. Pat. No. 4,700,051. The light radiator is an electrical heating resistor, which is heated to a much higher temperature (above 1500.degree. K. and preferably above 2000.degree. K.) than the standard heating resistor coils, which operate at a temperature below 1500.degree. K. and are referred to hereinafter as dark radiators, although they also operate in the red heat range. The light radiators are usually encapsulated in an inert gas atmosphere, e.g. in quartz glass bulbs or tubes and are in part provided with means to counteract or cancel out material evaporation, e.g. by a halogen filling.
A problem in connection with light radiators is the high positive temperature coefficient of resistance, which leads to extremely high and in part, inadmissible starting or transient currents on operating the light radiator alone. Thus, U.S. Pat. No. 4,700,051 provides for a series connection of a series resistor constructed as a dark radiator connected in series with the light radiator, which attenuates or damps the starting current and in operation as a dark radiator supplements the power delivered by the light radiator. Thus, the entire installed capacity can be distributed over the light radiator and the dark radiator, which is in particular advantageous in the hitherto conventional arrangement of the light radiator in straight bars for filling the entire heated zone.
In the case of very high light radiator power levels this still becomes problematical, because in spite of the series resistor the starting current is too high and consequently inadmissibly loads the mains.
An object of the present invention is therefore to provide a radiant element, which ensures an admissible starting current in light radiators for all the power ranges.
According to the invention this object is achieved by a temporarily acting attenuation device or damping device for acting on the light radiator.
The attenuation device preferably switches on the series resistor during the warming-up phase of the light radiator and advantageously is automatically switched off again following heating of the light radiator. It can operate as a function of the voltage drop at the light radiator which, due to the positive temperature coefficient, increases following its heating. It is possible to provide a threshold value switching device responding to the voltage drop and which can comprise a relay. However, it is preferably constructed as an electronic circuit which, on reaching a given voltage value, trips and brings about the disconnection of a series resistor. In order to avoid switching transients caused by voltage changes, the activation, i.e. the signal tripping the reconnection of the series resistor, can be initiated by the complete disconnection of the voltage, e.g. by a separate isolating switch.
However, the attenuation device can also comprise a delay circuit, i.e. a timing element, because conventionally the warming-up phase of a radiant element is very short and only last 1 to 2 seconds.
The series resistor can be a separate damping resistor, which is consequently disconnected during further operation. Due to the fact that the resistor is not exposed to permanent loads, it can be subject to a high loading and can therefore be small and simply constructed. However, it is preferably located in the vicinity of the radiant element, in order to be able to dissipate the heat produced by it and at the same time utilize the same.
Particularly in the case of two-circuit radiant elements, i.e. radiant elements having several, individually switchable heating zones, e.g. two heating zones concentric to one another, it is also possible to connect in series with the light radiators the dark radiators normally provided as permanent heating means in the form of series resistors across the attenuation device. Each of the dark radiators could be connected in series with a different light radiator than in the working circuit thereof, or they could also be switched in other combinations. Thus, e.g. the series resistors of both heating zones could be connected in series with a single light radiator or the dark radiator of one heating zone could be connected in series with the light radiator of the other.
In the case of all radiant elements, which incorporate a light radiator, the invention leads to a reduction of the starting current to an admissible value which, without the measures of the invention, can be in part about a power of ten over the working current and which would otherwise bring about inadmissible mains loading due to its short nature and surge-like occurrence. This more particularly applies if the control or regulation of the radiant element takes place by a timing cycle, e.g. by regulators or switches operating in power cycles of different relative on-times.
These and further features of preferred developments of the invention can be gathered from the claims, description and drawings. The individual features, either alone or in the form of subcombinations, can be realized in an embodiment of the invention and in other fields and represent advantageous, independently protectable constructions for which protection is hereby claimed.